Direct drive actuators employing active elements which are rods of magnetostrictive material are well-known. Examples of such actuators are disclosed and claimed in our published International Application WO 02/076141. The method of construction of these actuators means that although they deliver high force they have a physical profile that is unsuitable for some applications. Other active elements such as piezo can be incorporated into actuators that have a flat or narrow profile and may be suitable for many of the applications where a magnetostrictive actuator is unsuitable. However piezo actuators deliver comparatively low forces, require high voltages, about 100v, and are unsuitable for acoustic applications at frequencies below about 1 KHz. For these reasons piezo actuators may not be used. Higher force stacked piezo actuators are available but these are expensive, difficult to manufacture and tend to be unreliable. The height of the stack may also create an unacceptable profile. One potential solution to providing a high force, low profile actuator has been to use a flex-tensional envelope around an active element, as disclosed in USA4845688, that may be a magnetostrictive or piezo engine, but this is still too bulky for many applications.
Conventional axially-arranged actuators typically require an internally-mounted annular spring to provide the pre-tension required to optimise the performance of the active material, for example magnetostrictive material or piezoelectric material. It has been found through experimentation and trial that distortion of the output acoustic signal generated by such a device, particularly when miniaturised, can arise through the annular spring allowing a non-predictable extension to the driven face, resulting in an off-square output force which compromises the audio output.
Audio actuators of different construction produce different frequency bandwidths. Broader bandwidth has been achieved by having a variety of different actuators each driving a surface, or the same surface, separately. This invention describes different methods of combining features of different constructions within a single actuator to achieve broader bandwidth, and consequentially improved audio output, while reducing the overall cost of manufacture and installation. It is also known to combine different materials in a single actuator, for example piezo and magnetostrictive to create a specific output of force and frequency for a particular application.
In a magnetostrictive actuator it is well-known that the design of the coil and size of the magnetostrictive piece of material, amongst other things, influence the frequency response and volume output of the actuator on any surface. It is also well known that actuators can be constructed with a single stack of coils with magnets between the coils in the stack.